1. Technical Field
The present invention relates to a temperature controlled crystal oscillator that uses a crystal resonator (hereunder, referred to as “temperature controlled oscillator”), in particular, to a temperature controlled oscillator that detects the surrounding temperature of the temperature controlled oscillator.
2. Background Art
A temperature controlled oscillator maintains the operating temperature, in particular, of a crystal resonator, at a constant temperature, and is thereby capable, without causing a frequency variation dependent on frequency-temperature characteristics, of obtaining highly stable oscillating frequencies of the order of, for example, 0.1 ppm or lower, or 1 ppb. The temperature controlled oscillator may be applied, in particular, to a fixed station of a communication facility.
3. Prior Art
FIG. 10 and FIG. 11 are diagrams for describing a conventional example of a temperature controlled oscillator disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 2005-165630), wherein FIG. 10A is a cross-sectional view of the temperature controlled oscillator, FIG. 10B is a plan view of a circuit substrate, and FIG. 11 is a schematic circuit diagram of a temperature control circuit.
As shown in FIG. 10A and FIG. 10B, a temperature controlled oscillator 1 has a surface mount resonator 2, an oscillating circuit 3, and a temperature control circuit 4, and respective devices forming these are arranged on a circuit substrate 6 on a metallic base 5. Through the metallic base 5, there are passed lead wires 7a to 7d (the cross-sectional views thereof are omitted here in order to avoid complexity in illustration), and the circuit substrate 6 is held by the lead wires 7a to 7d. The lead wire 7a that serves as a ground is fixed in a through hole 8a of the metallic base 5 using, for example, silver braze 8, and is electrically and mechanically connected to the metallic base 5. The lead wires 7b to 7d, other than the ground 7a, that serve as a power supply terminal, an output terminal, and the like are insulated and connected with a through hole 9a of the metallic base 5 (so-called “airtight terminals”). A metallic cover 10 is joined with the upper surface of the metallic base 5 by means of resistance welding so as to seal-enclose the circuit substrate 6 therein.
The surface mount resonator 2 is configured such that an AT-cut crystal element (not shown in the diagram) is accommodated in a cross-sectionally concave-shaped surface mount resonator base composed of a laminated ceramic, and a resonator metallic cover is joined on the opening end surface of the resonator base. On the outer bottom surface of the surface mount resonator 2 (above the surface mount resonator 2 in FIG. 10A), there are provided mount terminals that electrically connect to this crystal element.
Moreover, the oscillating circuit 3 (the portion surrounded by the chain line 3 in FIG. 10B) comprises a capacitor and an oscillating amplifier arranged in an oscillating section, and it is of a Colpitts type circuit with the surface mount resonator 2 serving as an inductor component.
The temperature control circuit 4 (the portion surrounded by the chain line 4 in FIG. 10B), as shown in FIG. 11, has a thermistor 11 (the temperature characteristic thereof is negative) that detects the operating temperature of the surface mount resonator 2, a linear resistor 12 (the temperature characteristic thereof is positive) that detects the surrounding temperature of the temperature controlled oscillator 1, resistors 13A to 13C, an operational amplifier 14, a power transistor 15, and a heating resistor 16 that applies heat to the surface mount resonator 2. As shown in FIG. 10A, the thermistor 11, the power transistor 15, and the heating resistor 16, along with the surface mount resonator 2, are arranged on a principal surface of the circuit substrate 6 facing the metallic base 5, and are covered by a thermally conductive resin 17 so as to be thermally bonded thereon. As shown in FIG. 10B, the linear resistor 12 is arranged in a position distanced from the heating resistor 16 so as to facilitate detection of the surrounding temperature of the temperature controlled oscillator 1 (so as to improve responsiveness to the surrounding temperature).
As shown in FIG. 11, the linear resistor 12, the resistor 13A, and the thermistor 11 are arranged in series, with the linear resistor 12 connected to ground, and the thermistor 11 connected to a power supply voltage VCC. The power supply voltage VCC is voltage-divided by the thermistor 11, and the resistor 13A and the linear resistor 12, and the reduced voltage is taken as a control voltage. Moreover, the resistor 13B and the resistor 13C are arranged in series, with the resistor 13C connected to ground, and the resistor 13b connected to the power supply voltage VCC. The power supply voltage VCC is divided by the resistor 13B and the resistor 13C, and the reduced voltage is taken as a reference voltage.
As shown in FIG. 11, the operational amplifier 14 receives inputs of a reference voltage and a control voltage, and amplifies and then outputs the voltage difference between the reference voltage and the control voltage. The output of the operational amplifier 14 (differential voltage) is applied to the base of the power transistor 15, and the output current of the collector (collector current) is controlled by the input voltage of the base (base voltage and differential voltage). The heating resistor 16 is connected to the collector. The heating resistor 16 generates heat according to the collector current, and consequently the surface mount resonator 2 is heated while it is also heated by the heat generated by the power transistor 15. The temperature control circuit shown in FIG. 11 is an embodiment of the temperature control circuit disclosed in Patent Document 1 (Japanese Unexamined Patent Publication No. 2005-165630).
In a temperature controlled oscillator of such a conventional example, stable oscillating frequencies can be obtained even in those cases where the surrounding temperature of the temperature controlled oscillator 1 changes. The reason therefor is described below.
FIG. 12 shows the frequency-temperature characteristic of the surface mount resonator 2 having an AT-cut crystal element accommodated therein. This frequency-temperature characteristic draws a third order curve having a peak temperature in the vicinity of the temperature 85° C. on the high temperature side at or above the normal temperature 25° C. In FIG. 12, the horizontal axis represents the operating temperature of the surface mount resonator and the vertical axis represents the frequency deviation Δf/f where f denotes vibration frequency (resonance frequency) at the normal temperature 25° C. and Δf denotes a frequency difference with respect to the vibration frequency fat the normal temperature 25° C. As shown in FIG. 12, the vibration frequency changes with a change in the operating temperature of the crystal resonator, and accordingly, the oscillating frequency of the crystal oscillator also changes.
Consequently, the heat release temperature of the heating resistor 16 is controlled with the temperature control circuit 4 shown in FIG. 10B, to have the operating temperature of the surface mount resonator 2 at a constant temperature, thereby stabilizing the oscillating frequency. Specifically, the operating temperature of the surface mount resonator 2 is set such that the control voltage, with which the operating temperature becomes the temperature 85° C. for example, is preliminarily set to a voltage lower than the reference voltage. Thereby, the resistance value of the thermistor 11 becomes greater as the operating temperature of the surface mount resonator 2 drops, and consequently the control voltage drops. Accordingly, the differential voltage between the control voltage and the reference voltage becomes greater, the collector current flowing to the heating resistor 16 increases, and the amount of heat release of the heating resistor 16 becomes greater. On the other hand, if the operating temperature of the surface mount resonator 2 rises, the resistance value of the thermistor 11 becomes lower, the differential voltage and the collector current are consequently reduced, and the amount of heat release of the heating resistor 16 becomes lower. Therefore, the operating temperature of the surface mount resonator 2 is maintained at the temperature 85° C. and the oscillating frequency is maintained at a substantially constant frequency.
That is to say, the temperature control circuit 4 of the conventional example shown in FIG. 10A and FIG. 10B is such that, as described above, the thermistor 11 detects the operating temperature of the surface mount resonator 2, and the amount of heat released by the heating resistor 16 is controlled to thereby have the operating temperature of the surface mount resonator 2 at a constant temperature. However, even if the thermistor 11 has detected the operating temperature of the surface mount resonator 2, in the case where the surrounding temperature of the temperature controlled oscillator 1 is different, such as with normal temperature 25° C. and low temperature −30° C., the amount of heat release (heat release temperature) of the heating resistor 16 that maintains the operating temperature of the surface mount resonator 2 at 85° C. also needs to be made different depending on the surrounding temperature. For example, if the heating temperature is T1° C. when the surrounding temperature is the normal temperature 25° C., when it is −30° C., T2° C. (>T1° C.) needs to be satisfied. This is because, even when the amount of heat release of the heating resistor 16 is the same, if the surrounding temperature is different, the amount of heat absorbed by (released into) the surrounding atmosphere is different and also the amount of heat supplied to the surface mount resonator 2 is different.
Consequently, in the conventional example, as shown in FIG. 11, the linear resistor 12 that detects the surrounding temperature is connected to the resistor 13A in series. The linear resistor 12, as described above, has a positive temperature characteristic, and the resistance value increases as the temperature thereof rises. Therefore, if the surrounding temperature drops (for example, from normal temperature 25° C. to −30° C.), the operating temperature of the surface mount resonator 2 also drops. Furthermore, as described above, the resistance value of the thermistor 11 increases, and the resistance value of the linear resistor 12, which has a positive characteristic, becomes lower. Thus, in a case where the linear resistor 12 is added in addition to the thermistor 11, the control voltage increases and the differential voltage becomes greater compared to the case of only having the thermistor 11. Also, as shown in FIG. 10B, the linear resistor 12 is arranged in a position distanced from the heating resistor 16, thereby making the responsiveness to the surrounding temperature sensitive.
As a result, if the surrounding temperature of the temperature controlled oscillator 1 drops, the collector current and the amount of heat release of the heating resistor also increase, and the amount of heat release becomes dependent not only on the temperature detected by the thermistor 11 but also on the surrounding temperature of the temperature controlled oscillator. Needless to say, in contrast, even in a case where the surrounding temperature has increased, due to reverse action, the amount of heat release becomes dependent not only on the temperature detected by the thermistor 11 but also on the surrounding temperature. For this reason, even if the surrounding temperature changes within a temperature range, for example, from the normal temperature 25° C. to the temperature standard −30 to 85° C., the heating resistor 16 generates an amount of heat that maintains the operating temperature of the surface mount resonator at 85° C. and that differs according to the surrounding temperature, and consequently, the oscillating frequency at temperature 85° C. becomes more stable compared to the case of only having the thermistor 11.